Sudden cardiac death is the main cause of death in heart failure patients and is believed to be associated with cardiac arrhythmias. Cardiac arrhythmias can be classified in two types: tachycardic and bradycardic. Tachycardic arrhythmias include ventricular tachycardia and ventricular fibrillation. Bradycardic arrhythmias include sinus arrest and extreme sinus bradycardia. Extreme bradycardic episodes can lead to extreme ischemic cardiac conditions and, as a result, enhanced myocardial sensitivity and possible propagation of cardiac arrhythmias.
To manage bradycardia, many pacemakers have a programmed lower pacing rate. If the patient's heart rate drops below this rate, the pacemaker intervenes by delivering pacing pulses at the lower pacing rate until the natural or intrinsic heart rate is restored to at least the lower pacing rate. Typically, this lower rate is programmed to a level well below the intrinsic rate of the sinus node frequency. For example, for patients with an intact sinus node function, the sinus rate is often programmed to 60 pulses per minute (ppm). Sleep rates may be programmed at an even slower rate, e.g., 50 ppm.
Some single chamber pacemakers incorporate a hysteresis feature that gives the heart an opportunity to beat on its own before the pacemaker intervenes by increasing the base pacing interval by the hysteresis interval. Hysteresis refers to extension of the range of cardiac rates at which stimulation pulses are inhibited. Dual chamber pacemakers, however, typically do not incorporate a hysteresis feature. As a result, patients using dual chamber pacemakers may receive unnecessary cardiac stimulation that potentially interferes with natural heart activity by preventing depolarization of the sinus node.
A heart failure patient often has an intrinsic atrial rhythm, but exhibits periods of sinus node dysfunction, including sinus arrest and/or extreme bradycardia. During periods of sinus node dysfunction, the patient receives backup pacing at the programmed lower rate. Because the paced rate is slower than a typical baseline heart rate, e.g., 75-90 beats per minute, cardiac output is significantly reduced. For example, assuming a paced rate of 60 pulses per minute, cardiac output is decreased by approximately 30% relative to the baseline heart rate. As a result, for heart failure patients, the programmed lower rate is insufficient for maintaining heart and body perfusion requirements.
Table 1 lists patents that disclose pacemakers that deliver pulses at an increased rate in response to a detected rapid drop in heart rate. After a timed period elapses, the pacing rate is decreased toward the lower pacing rate.
TABLE 1U.S. Pat. No.InventorsTitle5,284,491Sutton et al.Cardiac Pacemaker with HysteresisBehavior5,501,701Markowitz et al.Pacemaker with Vasovagal SyncopeDetection and Therapy5,540,728Shelton et al.Pacemaker with Vasovagal SyncopeDetection5,676,686Jensen et al.Pacemaker with Vasovagal SyncopeDetection
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the techniques of the present invention.